1. Field of the Invention
The present invention relates to a color image capture element that is sensitive to light in the infrared region and to a color image signal processing circuit.
2. Description of the Related Art
Image capture elements, such as charge coupled devices (CCD) and complementary metal oxide semiconductors (CMOS) mounted in cameras, are arranged in two dimensions and comprise photoelectric conversion elements for converting light into electric signals. Since the photoelectric conversion element itself does not have any capability to distinguish color, color images are captured by performing color separation on the incoming light, such as with color filters, and performing photoelectric conversion on each color separated light. Color separation of the incoming light is generally performed by color filter layers of the three primary colors (RGB) or the complementary colors (YMC).
Furthermore, compared to the wavelength of light of approximately 380 nm to 780 nm that is visible to the human eye, an ordinary photoelectric conversion element detects light in a wavelength region of 380 nm to 1100 nm and also has a high sensitivity in the infrared region, which has a longer wavelength than visible light. When the photoelectric conversion element, which outputs signals corresponding to the intensity of the respective colors after color separation of the incoming light, also outputs light components in the infrared region, which is invisible to the human eye, true color reproduction cannot be achieved. Accordingly, an infrared-cut filter is disposed between camera lens and photoelectric conversion element so that infrared light does not enter the photoelectric conversion element.
Although the infrared-cut filter that is provided for true color reproduction cuts any light having a wavelength in the infrared region, it also simultaneously cuts approximately 10% to 20% of visible light. Therefore, the intensity of the visible light striking the light receiving surface of the photoelectric conversion element decreases, and as a result the S/N ratio of the output signal drops, thereby creating a problem inducing image deterioration. Particularly in European cultures, indirect illumination with incandescent bulbs having a large amount of infrared components in the emission spectrum is commonly used so that such indoor illumination is relatively dark. An improvement in sensitivity of the capture element is thus desirable to perform image capture at a high S/N ratio even under such low illumination.
Switchable monochrome/color cameras have been proposed wherein the output of the CCD is monitored so that when the CCD output level is large (such as during the day when the light is strong), an infrared-cut filter is inserted between the lens and CCD before color images are captured, and when the CCD output level is small (such as during the night when the light is weak), the infrared-cut filter is automatically switched to a dummy glass.
Furthermore, for capturing images under a variety of illumination light sources, a color image capture apparatus in general automatically adjusts the gain for the RGB signals so that the white balance is correct regardless of the type of light source under which the image was captured.
Higher sensitivity is desirable in color image capture apparatuses so that vivid color images can be captured even in dark environments. To achieve higher sensitivity, many methods are adopted, such as dropping the frame rate, opening the lens aperture, and raising the gain of the signal amplification. However, these methods cause problems, such as the resulting afterimage, shallower depth of field, increased noise, and so forth.
Consequently, a method was devised to remove the infrared-cut filter, which causes a loss in the visible light as described hereinabove, and efficiently guide the incoming light to the photoelectric conversion element. However, this still resulted in a problem making it impossible to achieve correct color reproduction. Furthermore, it has become desirable to remove the infrared-cut filter in the color image capture element also from the viewpoint of miniaturization and lower cost.